Parison oven

ABSTRACT

An oven for heating tubular parisons to a target or orientation temperature. A first plenum on one side of the oven and a second plenum on the opposite side of the oven, both exhaust to a common chamber disposed therebetween. The parisons are heated by a high velocity fluid above the target temperature in the zone defined by the first plenum and the chamber and the parisons are tempered by a lower velocity fluid at the target temperature in the zone defined by the second plenum and the chamber.

BACKGROUND OF THE INVENTION

This invention relates to blow molding of oriented hollow articles ofthermoplastic material and more specifically to an oven for heating theparisons to a predetermined target or orientation temperature.

In recent years, there has developed techniques whereby blow moldedarticles can be produced having biaxial orientation, which articles haveexceptional strength and highly desirable physical properties such asclarity. This technique involves forming of parison, cooling it to wellbelow its melting point, and thereafter heating it to orientationtemperature. The orientation temperature is just below the crystallinemelt point in the case of crystalline materials and 40° to 225° F. belowthe homogeneous melt point for amorphous materials. Such techniques aredisclosed in U.S. Pat. Nos. 3,288,317 and 3,390,426.

Ovens for heating such parisons can be found in U.S. Pat. Nos. 3,752,641and 3,740,868, both of which are assigned to the present assignee. Inovens of this type, parisons are disposed on pin structures carried by amovable chain through the oven. Parisons were carried along a pluralityof generally parallel paths in an upright position and hot air wasforced from the bottom of the oven and exhausted through the top. Animprovement was made in this type of oven in U.S. Pat. No. 3,801,623 andassigned to the present assignee in which an oven of this same generalconstruction was utilized whereby the heated air was blown across theoven from one side to another for more effective heating. Two specificproblems occurred: (a) it took considerable time to heat certainmaterials, such as polypropylene, and (b) there was a tendency for theouter parison wall in an axial direction to be hotter than thecorresponding inner parison wall in an axial direction resulting in theformation of a nonuniform container.

What is desired then is a parison oven which can reduce the time neededfor heating parisons and thereby increase production thereof, and anoven for more uniformly heating the parisons so that the peripheralwalls of the parisons have a temperature approximately equal tocorresponding portions on the interior walls of the parisons.

SUMMARY OF THE INVENTION

In accordance with this invention, an oven is provided for heatingtubular thermoplastic parisons up to a target temperature approximatelyequal to its orientation temperature. The oven includes transportingmeans for carrying the parison through the oven. Furthermore, on onewall of the oven is means defining a first plenum chamber for supplyinga first heating fluid in a cross flow direction. On the opposite side ofthe oven is means defining a second plenum chamber for supplying asecond heating fluid in a cross flow direction opposed to said firstheating means. Disposed between the first and second plenum is anexhaust chamber. The exhaust chamber and the first plenum define a firstheating zone and the exhaust chamber and the second plenum define asecond heating zone.

In the first zone, parisons are quickly heated by a heating fluidsupplied at high velocity and at a temperature higheer than the targettemperature. In the second zone, the parisons are tempered by a lowervelocity heating fluid supplied to said second zone at a temperatureapproximately equal to the target temperature. Therefore, in the firstzone the parisons are quickly heated and in the second zone, theparisons are tempered to enable equalization between the temperatures ofthe periphery of the parison and those correspondingly on the inside ofthe parison.

What is disclosed then is an oven which substantially decreases theheating time required for the parisons, and furthermore, provides moreuniformly heated parisons for the subsequent blow molding operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a plan view of the blow moldingmachine utilizing the oven formed in accordance with the principles ofthe present invention; and

FIG. 2 is a view taken along lines II--II in FIG. 1 and on an enlargedscale.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, there is generally shown an oven 10. The oven10 is supported on a base structure 11. The oven 10 is generallycomprised of an outer wall member 13 and an inner wall member 15.Disposed therebetween is a suitable insulating wall member 16.

A first plenum 18 is disposed on a first wall 17 of the oven 10. Theplenum 18 has a wall member 19 facing the interior of the oven whichincludes a solid bottom portion 20, a solid top insulated portion 21 anda perforated plate middle portion 22. The perforated plate 22 isfastened to the bottom and top portions 20 and 21 by any suitable means,not shown. An inlet opening 24 is disposed in the bottom wall of theplenum 18.

On a second wall 26 of the oven 10, there is disposed a second plenum28. A wall 29 of the plenum 28 facing the oven includes a solid bottomportion 30, a solid top portion 31, and a middle portion comprising aperforated plate member 32. The perforated plate 32 is securely fastenedto the bottom and top portions 30 and 31, respectively, by any suitablemeans, not shown. In the bottom wall partially defining the secondplenum 28, is a suitable inlet opening 34.

An exhaust chamber 36 is disposed between the first plenum 18 and thesecond plenum 28. The exhaust chamber 36 includes a wall 39 extendingbetween the top and bottom of the oven 10. The wall 39 includes a bottomportion 40, a top portion 41, and an intermediate portion comprising aperforated plate 42, fastened thereto. The wall 39 of the exhaustchamber 36 and the wall 19 of the plenum 18 define a first heating zone44. Another wall 46, which defines another portion of the exhaustchamber 36, includes a bottom portion 47, a top portion 48, and anintermediate portion 49, which includes a perforated plate. Theperforated plate 49 is firmly secured to the bottom and top portions 47and 48, by any suitable means, not shown. On the bottom of the exhaustchamber 36 is disposed an outlet opening 51, which is disposed in thebottom wall thereof. The wall 46 of the exhaust chamber 36 and the wall29 of the second plenum define a second heating zone 53.

The parison transporting means includes a chain 55 which defines sixpasses through the first heating zone 44 and two passes through thesecond heating zone 53. A driving roll 57, disposed outside of the oven10, drives the chain 55 around idler rolls 58 through 67 insubstantially back and forth parallel passes.

As best seen in FIG. 2, the chain 55 is enclosed within supportstructures 69 which are fastened to the bottom of the oven 10. On theupper portion of support structure 69, is a friction member 70 disposedon alternate sides of the support structures 69 depending on thenumbered pass. Conical shaped pin members 73 are carried by the chain 55at spaced intervals and rotatable star wheels are secured thereto. Asthe chain 55 is driven by the roll 57, the star wheels 72 engage thefriction members 70 to rotate the star wheels, pin members 73, and anyparisons disposed thereon (shown in phantom).

Electrical resistance strip heaters 75 are disposed between the last twochain passes in the first heating zone 44. Strip heaters 76 are disposedbetween the two passes in the second heating zone 53. After the lastpass, in the second heating zone 53, the chain continues to the pickingstation 78 and goes between strip heaters 79, which are generally usedfor oval shaped bottles.

Two or more parisons can be picked at the picking station 78 by a pickerarm (not shown) and carried to the mold station 81. At the mold station81, there is shown two oval shaped cavities 82 and 83 for blowing thetubular shaped parison into an oval container, although any suitableshaped container can be formed.

The chain 55 continues around idler roll 67 and leaves the oven 10. Anyparisons left on the star wheels 72 and pins 73 are removed at themissed parison removal station 84. The chain then continues to drivingroll 57 where parisons are fed onto the star wheels and pin members atloading station 86 by means, not shown.

Heated air is circulated by centrifugal blower 88 through heater 89 tothe inlet opening 24 of first plenum 18 where the heated air is forcedthrough the perforated plate 22 of the wall 19. The air is forced acrossthe first heating zone 44 and is removed through the perforated plate 42of the wall member 39 partially defining the exhaust chamber 36. Air isalso known through centrifugal blower 91, and heater 92 to the inletopening 34 of second plenum 28. The air is forced through the perforatedplate 32 of the wall 29 and flows across the second heating zone 53. Theair exits through the perforated plate 49 of the wall 46 partiallydefining the exhaust chamber 36. The air is then exhausted through theoutlet opening 51 of the exhaust chamber 36 and is recirculated to bothcentrifugal blowers 88 and 91. Any necessary makeup air if fed inthrough line 94.

As previously indicated, the main problems experienced in the previousovens were the length of time required to heat parisons, andnonuniformity in the heating of the parisons. It has been unexpectedlydiscovered that by dividing the oven into two heating zones, thevelocity and temperature of the heating fluid in a first zone can beincreased substantially above prior velocity and temperature of theheating fluid. Furthermore, by increasing the velocity, there will be acorresponding decrease in the scale of stray air or eddy currents whichenables a more uniform heating of the parisons. Another effect ofincreased velocity is less heat loss (per lb. of air flowing through theoven) due to heat exchange with the oven walls. There is also areduction in any natural convection air currents and consequently, amore uniform and constant air temperature throughout the heatingchamber. Finally, by adding a second zone or tempering zone, a moreuniform and equally heated parison can result. This unexpectedlysubstantially reduces cycle time and also forms a much more uniform,clearer, and stronger container.

It is desirable to reheat parisons just below the crystalline melt pointin the case of crystalline materials and between 40° to 225° F. betweenthe homogeneous melt point for amorphous materials. This is commonlycalled the orientation temperature. The final temperature the parisonsare heated to is in this instance slightly higher than the orientationtemperature since there will be some small temperature loss in movingthe parisons from the picking station 78 to the mold 81. In the priorart ovens, it was common to have the heated air blown through theheating zone at approximately 800 cubic feet per minute. Because of thelarge volume of the single zone, plus the relatively poor air flow,there was substantially stray eddy currents and nonuniform temperaturesin the oven. Temperature variations at the same point in the oven wereas high as 20° F. The outer walls of the parisons were subjected tolocalized hot spots and the inner walls of the parisons, because of lowthermal conductivity, had substantial temperature variations at an axialpoint versus the corresponding radial points on the outer walls.

According to the principles of this invention, the oven was divided intothe two zones 44 and 53. High velocity and a higher temperature heatingfluid is introduced in the first zone and high velocity fluid at thetarget temperature equalizes or tempers the parisons in the second zone.An increase of the velocity of the heating fluid in zone 44 greatlyminimizes the eddy currents, thereby forming an atmosphere ofsubstantially more uniform temperature. For example, in zone 44 thevelocity of the heating fluid was increased from about 800 cubic feetper minute to about 3,000 cubic feet per minute.

However, there is a theoretical upper limit to the increase in velocityof the heating fluid as well as a practical cost limitation. Theparisons are supported on one end by the star wheel 72 and conicalmember 73. This in effect is a cantilevered structure. By increasing thevelocity of the fluid, there is a greater deflection on the free end orunsupported end of the parison. If there is sufficient deflection at thefree end of the parison, the parison picking arm at the picking station78 will not effectively pick the parison, and when a deflected parisonis deposited in the mold 81, a defective container is formed. The amountof deflection which is acceptable varies depending on the material used,the diameter and thickness of the parison, and the height of theparison. The oven is designed so that the velocity of the heating fluidforced from the first plenum 18 into the first zone 44 is just below thevelocity causing sufficient deflection to the parison so that anunsatisfactory container is formed, this upper limit hereinafter beingdefined as the "distortion velocity." The concept of distortion velocityis also applicable to, for example, a parison supported at both endswhereby the deflection occurs in the middle of the parison.

Another theoretical upper limit to air velocity is the "tipping"velocity or velocity which the parison would tip over before actualdistortion occurred. Therefore, either the distortion velocity or thetip over velocity is the theoretical upper limit, depending on the typeof material and the size of the parison used.

By increasing the velocity of the heating fluid to a velocity less than,but approaching the distortion velocity or tipping velocity, there is asubstantial reduction in scale of eddy currents, and a marked reductionof temperature variation. The heating rate is also increased and thereis a corresponding reduction in cycle time. A more uniform temperaturein the oven heats the parison more uniformly resulting in an improvedblow molded container.

It has also been found that by increasing the velocity of the heatingfluid at the same temperature, there is even a greater temperaturevariation between the inner and outer walls of the parisons. Thetemperature in the second zone is maintained at the target temperatureand the heating fluid is supplied at a rate to minimize air currents andeffect proper heat transfer from the outer surface to the inner surfaceof the parison to obtain a temperature leveling.

It has also been discovered that the heat transfer effectiveness can beincreased by supplying heating fluid of a higher temperature than waspreviously supplied to the first zone 44. The parisons will be heated tojust below the distortion temperature, i.e., that temperature which ifthe material is heated to, will cause sufficient damage to at least aportion of the parison surface so that an unsatisfactory container isformed. The upper limit of the distortion temperature will be just belowthe crystalline melting point for crystalline materials and just belowthe homogeneous melt point for amorphous materials. The lower level forthis temperature will be just above its target temperature.

As the parisons are heated in the first zone by the high velocity, hightemperature heating fluid, nonuniform localized hot spots on the outerperiphery of the parisons result. Portions of the outer walls are abovethe target temperature and the inner walls of the parisons are below thetarget temperature. The parisons are then conveyed to the second zone 53which is maintained by heated fluid from the second plenum at the targettemperature. The heating fluid blowing across the parisons at the targettemperature provides a tempering zone so that equalization oftemperatures or a temperature leveling between the inner and outer wallscan be achieved, i.e., hot areas on the outer walls, cool and the coolinner walls increase in temperature, both walls approaching the targettemperature.

Additional heating means may be used, such as the strip heaters 75 and76 to program the parisons for forming certain shaped containers. In thefifth and sixth passes, certain portions of the parisons in an axialdirection can be additionally heated by means of strip heater 75. Thislocalized heating in an axial direction is continued in the seventh andeighth passes in the second zone 53 by means of strip heater 76. Thetempering zone in the second chamber 53 also tends to equalize thetemperature of the parisons even when programmed, i.e., the temperatureat a given height in an axial direction will tend to be hotter on boththe internal wall and the exterior wall depending on the placement ofthe programmed heater. Finally, heaters 79 can be utilized to heatparisons on each side which can be specifically utilized for oval shapedbottles. The parisons going through the area defined by the heater 79 donot rotate in contrast to the continuing rotation of the parison throughthe first and second zones 44 and 53, respectively.

EXAMPLE 1:

In a previous machine, parisons made of polypropylene were utilizedhaving a 6 inch axial length, 1.3 inch outside diameter, and a 0.175inch wall thickness. The oven temperature was approximately 326° F. andthe air volume going through the oven was 800 cfm. The resultant bottle,with five oven passes, took 34.5 minutes to heat, and provided six toseven parisons per minute to the blow molding station.

EXAMPLE 2:

An oven formed in accordance with the principles of the presentinvention and utilizing polypropylene parisons of substantially the samedimensions and characteristics as those given in Example 1, have thefollowing parameters. In the first heating zone, the air was provided at3,000 cfm at 350° F. The heating air was 326° in the tempering zone at aflow rate of 1,000 cfm. There were six passes in the primary zone andtwo passes in the tempering zone. Eighteen parisons per minute wereprovided to the blow molding station. Besides having a one-thirddecrease in heating time, and an increase in the amount of parisonsprovided per minute, the resulting molded containers showed a markedincrease in homogeneity, clarity, and material distribution because ofthe increased accuracy and uniformity in heating.

The above examples are given for the purpose of showing the magnitude ofimprovement between the prior art oven vs. the improved oven, but ofcourse the flow rates, temperatures, time cycles, etc., willsubstantially change depending upon the type of material, wallthickness, and size of parisons used.

In summary then, it can be seen that the parison oven disclosed hasdivided the functions between quickly heating the parisons and uniformlyheating the parisons. In the first zone, the parisons are heated at atemperature above the target temperature, but below the melting point ofthe parisons. The air velocity in the first zone is slightly below thedistortion velocity. The higher velocity fluid also tends to miminizeeddy currents and flow distribution, therefore also providing moreuniformly heated parisons. The parisons are transferred to the secondzone whereby a lower temperature fluid substantially equal to the targettemperature, is provided to equalize the wall temperatures between theinner and outer walls of the parisons. The resulting oven heats theparisons to the target temperature more quickly and more uniformly thanpresently possible.

While only one embodiment has been shown, it will be obvious to thoseskilled in the art that various modifications can be made withoutdeparting from the scope and spirit of the invention.

What is claimed is:
 1. An oven for heating tubular thermoplasticparisons to a target temperature including:means for transporting saidparisons through the oven; means for defining a first plenum in saidoven; means defining a second plenum in said oven; said first plenumbeing disposed on one wall of the oven and said second plenum beingdisposed on a second wall of the oven, said second wall being oppositesaid first wall; means defining an exhaust chamber disposed between saidplenums; said first plenum and said chamber defining a first heatingzone; means for supplying heating fluid to said first zone from saidfirst plenum for fast heating of said parisons; said second plenum andsaid chamber defining a second heating zone; means for supplying heatingfluid to said second zone from said second plenum for tempering saidparisons and substantially equalizing the temperature of the parisons.2. The oven recited in claim 1 and including means to supply heatingfluid to said first zone at a temperature above the target temperatureof said parisons, and means to maintain said heating fluid in said firstzone at a temperature above the target temperature of said parisons. 3.The oven recited in claim 1 including means to supply said heating fluidto said second zone at a temperature approximately equal to the targettemperature of the parisons.
 4. The oven recited in claim 1 andincluding means to maintain the heating fluid supplied to the chamber ata velocity slightly below the distortion velocity of the parisons. 5.The oven recited in claim 1 and including means to provide the heatingfluid to the chamber at a velocity slightly below either the tippingvelocity or the distortion velocity of the parisons, whichever velocitybeing lower in magnitude.
 6. An oven for heating hollow thermoplasticparisons having inner and outer walls to a target temperatureincluding:means defining a first plenum in said oven, said first plenumcooperating with said oven to define a first heating zone; meanssupplying heating fluid at a temperature above the target temperaturefrom said first plenum in said oven to said first heating zone forheating at least a portion of said parisons to a temperature above saidtarget temperature in said first zone; means defining a second plenum insaid oven, said second plenum cooperating with said oven to define asecond heating zone; means supplying heating fluid at the targettemperature from said second plenum in said oven to said second heatingzone, for tempering said parisons.
 7. The oven recited in claim 6 andfurther including means to maintain the temperature above the targettemperature and below the melting point of the parisons.
 8. Thestructure recited in claim 6 and further including means to transportparisons from said first to said second zone.
 9. An oven for heatinghollow thermoplastic parisons to a target temperature including:means toheat fluid to at least said target temperature of the parisons; meansdefining a first plenum, said first plenum cooperating with said oven todefine a first heating zone; means defining a second plenum in saidoven, said plenum cooperating with the oven to define a second heatingzone; means to supply said heating fluid from said first plenum to saidfirst zone at a velocity slightly below the distortion velocity of theparisons.
 10. The oven recited in claim 9 wherein means are provided tosupply the heating fluid at a temperature slightly below the meltingtemperature of the heated parisons and means to supply heating fluidthereafter to the parisons at the target temperature to equalize thetemperature of the parisons.
 11. The oven recited in claim 9 and furtherincluding meansto supply heating fluid at a temperature slightly belowthe melting temperature of the parisons to said first heating zone; andmeans to supply heating fluid at the target temperature to said secondheating zone.
 12. The oven recited in claim 9 and including means tosupply fluid to the second heating zone at a velocity which is below thevelocity being supplied to the fluid in the first heating zone.
 13. Theoven recited in claim 9 and further including additional heating meansto program the parisons.
 14. The oven recited in claim 9 wherein meansare provided to maintain the velocity of the heating fluid below thetipping velocity of the parisons.